1. Field of the Invention
The present invention relates to a composite dielectric thin film capable of high dielectric constant, low leakage current characteristics, and high dielectric breakdown voltage while being deposited at a room temperature, a capacitor and a field effect transistor (FET) using the same, and their fabrication methods.
2. Description of the Related Art
Recently, development of a transistor using oxide semiconductor (ZnO, In2O3, SnO2, InGaZnO4, and the like) as a channel layer (which is also referred to as a ‘channel region’, hereinafter) has been actively progressing. In particular, research on application of oxide semiconductor to a flexible electronic element and a transparent display based on the excellent semiconductor properties is ongoing.
Development of such flexible transistor has a restriction that the temperature of a fabrication process should be very low in consideration of stability of temperature at which a plastic substrate can tolerate. In addition, for its application to a substantial mobile circuit (namely, a circuit to which power can be supplied by using a battery), a driving voltage of a transistor element should be lowered to below 5V. In short, two factors of a low temperature process and a low voltage driving are critical in selecting a semiconductor channel layer and a gate insulating film of the transistor.
In order to obtain a low driving voltage, namely, a high capacitance value, of the transistor, a gate insulating film of high dielectric constant or a thin gate insulating film should be used. However, the plastic substrate generally has a rough surface (surface roughness: Rms˜3 nm or larger). If a low voltage driving transistor is fabricated by forming a thin gate insulating film on the rough substrate, a pin hole would be easily formed in the fabrication process or an insulation breakdown occurs at a low voltage due to a non-uniform step coverage, shortening the long life span and degrading stability of the element.
To overcome such problem the gate insulating film of 200 nm or thicker needs to be deposited, and in order to obtain a sufficiently low driving voltage even with a gate insulating film with a thickness of 200 nm or larger, a dielectric constant 15 or larger should be secured. In particular, in order to fabricate a gate insulating film on the plastic substrate to provide flexibility, a room temperature process is essential.
The organic insulating film is advantageous in that it can be easily fabricated at a low temperature, but its dielectric constant value is low by 3 to 6. Silicon dioxide (SiO2 with a dielectric constant 4) or silicon nitride (SiNx with a dielectric constant 7) commonly used for a gate insulating film of a polysilicon (Poly-Si) transistor or amorphous Si transistor has relatively low dielectric constant values. Although SiO2 and SiNx have high breakdown characteristics based on the stable leakage current characteristics, it needs to be deposited at a high temperature of 200° C. or higher, so there is a restriction in directly depositing SiO2 and SiNx on the plastic substrate.
In an effort to overcome such problem, there has been attempted to deposit high dielectric gate insulating film, namely, tantalum oxide (Ta2O5), strontium titanate (SrTiO3), yttrium oxide (Y2O3), zirconate (ZrO2), barium strontium titanate ((Ba,Sr)TiO3), barium zirconate titanate ((Ba,Zr)TiO3)[C. D. Dimitrakopoulos, S. Purushothaman, J. Kymissis, A. Callegari, and J. M. Shaw, Science 283, 822 (1999)], hafnium oxide (HfO2) [K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature 432, 488 (2004)], Alumina (Al2O3), titanate (TiO2), Bi1.5Zn1.0Nb1.5O7 [I. D. Kim, M. H. Lim, K. T. Kang, H. G. Kim, S. Y. Choi, Applied Physics Letters, 89, 022905, 2006], and the like, at a room temperature so as to be used as a gate insulating film.
When the above-mentioned insulating film is deposited at a room temperature, a relatively high dielectric constant value of 10˜25 can be obtained but with a high leakage current and low breakdown characteristics. Thus, if voltage of 10V or higher is applied to the insulating film, the insulating film would be broken. Thus, the high dielectric thin films fabricated at the room temperature can be hardly used as a dielectric thin film for a capacitor or a gate insulating film for a transistor.